Indicating lights have been provided to lines and machine and equipment in factories to indicate an operation status, an abnormality, a fault, etc. of the lines and equipment.
Some of the recent indicating lights adopt a light emitting diode (LED: an element that emits light by flowing a current through a PN junction of a semiconductor) (hereinafter, referred to as the LED indicting light).
FIG. 5 shows perspective views of the LED indicating lights. FIG. 5(a) shows a single-stage type having an indicator in one stage, and FIG. 5(b) shows a multi-stage type having indicators stacked in more than one stage. The multi-stage type can emit light in more than one color, which allows a current status to be judged by color discrimination of emitted light.
When the LED indicating light fails, the status of the lines or equipment cannot be judged, and therefore, it is necessary to monitor the occurrence of an abnormality in the indicator of the LED indicating light.
There has been known a technique of detecting a disconnection in a lamp or a disconnection in the indicator by constantly flowing a minimal current too small to turn ON the lamp (see Japanese Laid-open utility Model No. 5-038703 (1993)). However, this technique is intended for lamps having filaments.
The above method of constantly flowing a minimal current cannot be adapted to the LED indicating light, because the LED turns ON with such a minimal current.
It is therefore an object of the present invention to achieve a circuit allowing a fault diagnosis in the LED indicating light while making the turning ON of the LED unperceivable by human sense.